Cutaneous malignant melanoma (CMM) is the most lethal form of skin cancer, and its incidence has been increasing at an alarming rate in the U.S. and around the world. Exposure to damaging ultraviolet radiation (UVR) is clearly a risk factor for the disease, borne out by the recent linkage between tanning bed usage and dramatically increased melanoma incidence in young females. Better understanding of the mechanisms by which CMM progresses to metastatic forms is likely to provide important new therapeutic approaches for improving survival rates. The study of metastasis suppressor genes, such as NM23-H1 (NM23-M1 in mouse), has provided new insights into molecular mechanisms underlying melanoma metastasis. This application builds on our novel observations that NM23-H1 is a 3'-5' exonuclease which exerts proofreading activity on DNA in vitro, that its 3'-5' exonuclease activity is required for metastasis suppressor function, and that the protein promotes repair of UVR-induced lesions in DNA. In addition, transgenic mouse models developed for this project recapitulate for the first time the metastasis suppressor function of NM23-H1 in UVR-induced CMM. Our findings support a model in which loss of NM23-H1 expression results in compromised genomic integrity and mutations that drive malignant progression in CMM. In Specific Aim 1, the contributions of NM23-M1 and NM23-M2 to UV radiation-induced metastatic melanoma will be determined in transgenic mouse strains deficient in expression of one, or both, of the isoforms. In Specific Aim 2, the molecular mechanisms underlying promotion of DNA repair pathways by NM23 proteins will be elucidated, and the impact of perturbing those pathways on metastasis suppressor activity assessed in cell culture and tumor cell explanation model systems. Specific Aim 3 will apply the powerful technologies of massively parallel sequencing for whole genome sequencing and RNA-seq to primary and metastatic melanomas to identify candidate metastasis-driving genomic alterations and gene expression profiles. The proposed studies will provide the first systematic analysis of specific DNA repair and related pathways through which a metastasis suppressor gene opposes malignant progression, as well as the first application of a transgenic mouse model to the study of metastasis suppressor function in CMM. Our studies should provide novel and critical insights that can be pursued to combat melanoma in its advanced and lethal forms.